First, as far as I know VaM is not optimized to use your full CPU power, especially the physics engine (or parts of it) only runs on a single core IIRC. That's why the CPU usually is the limiting factor when it comes to more powerful (where there's more physics envolved) scenes. And that's why you'll probably never see VaM using your CPU to its full power -- a single core might get 100%, but all of your CPU? I doubt that...
Second: looking at your GPU -- it's at 100%. So why should the CPU use more of its power to "feed stuff" into your GPU if your GPU can't handle any more?
At least that's how I understand it, please correct me if I'm wrong.